Thermally stimulated processes are documented in the literature (Chapter 10 in the ACS Book Polymer Characterization, "Characterization of Polymers by Thermally Stimulated Current Analysis and Relaxation Map Analysis Spectroscopy, by J. P. Ibar, et al., Polymer Characterization Advances in Chemistry Series No. 227, Edited by Clara D. Craver and Theodore Provder; and for the TSCR: Chapter in the ACS Book Polymer Characterization, "Thermally Stimulated Creep for the Study of Copolymers and Blends" by Philippe Demont, et al.). The aim of such processes, which will be examined in greater detail hereinafter, is to understand the behavior of materials by studying the relaxations and internal motions which take place in order to optimize their mechanical, electrical, magnetic, etc., performances.
More generally speaking, the recovery process of a system applies to the phenomenon of recovering its initial state, after the application of a deformation has taken the system out of equilibrium. The recovery process is stimulated by a (linear) temperature increase, or can occur isothermally over time. Relaxation phenomena in materials during recovery are the results of internal motions due to disturbances either of a mechanical, electrical, magnetic or electromagnetic nature. Materials processed in industry have physical properties which depend on the ability to have local motion within the internal structure irrespective of whether this motion occurs at the level of the molecules, the atoms, or the macromolecules (for polymeric materials), or at the sub-atomic level. Deformation at one level or the other depends upon the type of excitation field involved to bring the material out of its equilibrium state at a given temperature.
Essentially, three types of methods for studying relaxation phenomena and resonances can be distinguished: (1) resonance methods, (2) damping analysis methods and (3) heat stimulated methods. In the resonance method, the material is subjected to a periodic excitation at a fixed frequency of a mechanical, electrical or magnetic nature at a determined temperature and fixed pressure. The periodic excitation frequency can be adjusted to enable the determination of the resonance frequency for this temperature and pressure. The frequency of resonance corresponds to the frequency of the internal motion occurring under these conditions. An alternative method, which is frequently used, consists in subjecting the material to an excitation at a determined frequency and programming a variation in temperature. When the temperature reaches a level capable of allowing the internal movements sought to be characterized, a resonance peak for the selected excitation frequency is observed. It is possible to operate at various (fixed) frequencies and thus analyze the dependence between frequency and temperature which provides access to the mechanism responsible for internal motion under investigation.
In many instances, the internal motion is kinetically controlled, and the variation in the resonance peak frequency (fm) varies with the maximum temperature of the peak Tm, and the results are often collected as the ln (fm) versus 1/Tm, a so-called Arrhenius diagram (Tm is in degrees Kelvin and ln is the natural logarithm). The linearity of the Arrhenius line is indicative of an activated phenomenon. The slope of the straight line in the Arrhenius diagram is related to the activation enthalpy of the process due to internal motions and the intercept is proportional to the activation entropy, i.e., to the jump frequency between the activated states allowing potion. By determining the values of the entropy and enthalpy, one can determine the origin of the movements occurring inside the material irrespective of their origin, whether it is viscous, atomic or sub-atomic. Mechanical deformation fields allow movements of the viscous type to occur in the material (so does ultrasonic excitation) and electrical fields (voltages) applied to the material allow the study of motions related to the electronic interactions between the atoms inside the material. The sub-atomic movements are delocalized by applying electromagnetic excitations.
In the characterization processes which are the subject of this invention, the temperature program is always the same regardless of the origin of the excitation, and consists of exciting the material at a particular temperature, then quenching it, interrupted by partial isothermal relaxation if necessary, and finally heating it up linearly to "develop" the response to the excitation stage during a thermally stimulated return to equilibrium.
The analysis methods using damping in the material consist in the application of a deformation of the material for a given length of time at a given temperature, cutting off the source of the excitation and analyzing the return to equilibrium (recovery curve) at that temperature by recording the freely oscillating damping curve. The equation of the recovery curve gives direct access to the damping factor at that corresponding temperature. The frequency of the oscillation and the damping factor relate to internal friction, and provide the relaxation time at the corresponding temperature. The frequency of oscillation and the damping factor vary with the temperature at which the material is being deformed. This enables the determination of the damping factor at different frequencies and different temperatures. As above, the origin of the internal motions may be found by studying the corresponding Arrhenius diagrams in plots of log of EQU ln (fm) vs. 1/Tm
The so-called "thermal stimulated" methods comprise purely calorimetric methods and methods combining the influence of temperature and a "stimulant" variable which may be a mechanical, electrical or an electromagnetic variable. Differential scanning calorimetry (DSC) consists in comparing the calorific energy flux supplied to two crucibles located in the same thermostatic atmosphere, a device in which one of the two crucibles contains the material to be analyzed. The temperature in the chamber may be programmed to increase, decrease, or to stay constant (isothermal mode). In a DSC the calorimeter is servo regulated in such a way that the temperature of the two crucibles is exactly the same. The variable energy flux supplied or subtracted from the crucibles is recorded as the temperature of the cell varies, or as a function of time under isothermal conditions. Differential Thermal Analysis (DTA) is a slight variant of this microcalorimetric DSC process, for which the fine difference in the temperature between the two crucibles is recorded as a function of the cell temperature. The difference in the temperature between the two crucibles changes when there is an alteration in the physical structure or in the physical and/or chemical structure resulting in the variation in enthalpy within the material. In a DSC analysis, the energy flow differential to maintain the two pans at the same temperature is recorded, and a peak is observed when there is a modification in the thermodynamic state of the material. The peak characteristics relate to the state of the material, and transcribe the extent of internal motion and local reorganization, for instance due to molecular relaxations. Differential scanning calorimetry is a rapid and streamlined method of determining phase transitions in materials, for example in order to determine fusion and solidification temperatures, and the glass transition temperature in the case of the amorphous phase of non-crystalline or semi-crystalline materials. It should be noted that in this characterization technique, temperature essentially plays two roles, that of stimulator by contributing thermal energy capable of initiating activated internal motions, and that of sensor, by comparative measurements of the temperature or the flow of energy of the two crucibles, one containing the material to be characterized.
One variant of this process consists in obtaining calorific heat capacity curves as a function of the temperature at different atmospheric pressures.
Atmospheric pressure plays an important role with respect to the kinetics of relaxation phenomena. It is presently known that an increase in pressure is accompanied by a restriction of internal movements, which is observed in differential microcalorimetry by an increase in the temperature at which the internal movements are released during a thermal analysis. Apparatus currently marketed enable microcalorimetry curves to be obtained at pressurized atmospheres. The pressure remains constant during the heating or cooling cycle of these analyses. It is one of the characteristics of the present invention to provide means to submit the crucibles and their content to a pressure history treatment to enable the fine characterization of internal motions inside the material under investigation.
A further important type of analytical instruments for measuring internal movements in materials by the thermal-stimulated effect is described in the works of several authors, and concerns thermal-stimulated current techniques (TSC), and thermal-stimulated creep techniques (TSCR). These techniques are relatively original with respect to the previously described techniques. In these techniques temperature plays the role of developer while the external variables imposed during the excitation stage play the role of "marker".
In a variant of the process, described in further detail hereinafter, temperature also plays the role of "filter" for the relaxation times; this is the "thermal-windowing" filtering method. The aim of the excitation, in the form of a mechanical, electrical or magnetic field, etc., imposed on the material at a given temperature, is to induce orientation, or more generally to cause an imbalance in the system, by the effect of the field on the free activation energy value. The field intensity imposed remains fixed for a given time, the time for the new state of equilibrium to establish itself, and the temperature is lowered very quickly (tempering) to a temperature at which the new thermodynamic state of the material is no longer able to modify itself, for kinetic reasons; consequently a "frozen-in picture" of the state obtained at high temperature is produced. Analysis by the thermal stimulated effect consists in suppressing the field at low temperatures and reheating the material, which is now free of all stresses, and in so doing freeing up the internal motions which are thermally activated to allow their return to equilibrium. The kinetics for the return to equilibrium, induced by the temperature, can be analyzed quantitatively and is a function of the processing parameters of the material and its chemical structure. It is also a function of the morphology.
The thermal stimulated effect reveals all the relaxation modes occurring in a global manner. If the local motions inside the material are not simple in the sense of a pure relaxation of the Debye type, or when there is a large degree of interactive coupling between the relaxation modes responsible for the global response of the material, it is then very difficult to attribute to the recovery curve any particular local motion occurring in the material. Since the entire response of the material to a given excitation is global, it is generally essential to deconvolute the global response and define the relaxation time distribution, corresponding by analogy to different coupled resonators. The coupling between the elementary modes of relaxation is subject to a specific kinetics, itself a function of structural, chemical and morphological parameters. The description of the elementary modes, their thermo-kinetic characteristics (activation energy and entropy) and the description of the coupling is essential for understanding the macroscopic properties of materials. The TSC (thermally stimulated current) and TSCR (thermal stimulated creep processes) are thermal stimulated techniques which use the application of a field, either electrical (for TSC) or mechanical (for TSCR) at a given temperature in order to orient the dipoles in the material (TSC) or the chain segments (TSCR), with the aim of disclosing their individual existence when heated up in a controlled manner after cooling, and after the application of the field has been removed.
The two techniques, TSC and TSCR, have been described in the literature (for the TSC: Chapter 10 in the ACS Book Polymer Characterization, "Characterization of Polymers by Thermally Stimulated Current Analysis and Relaxation Map Analysis Spectroscopy, by J. P. Ibar, et al., Polymer Characterization Advances in Chemistry Series No. 227, Edited by Clara D. Craver and Theodore Provder; and for the TSCR: Chapter in the ACS Book Polymer Characterization, "Thermally Stimulated Creep for the Study of Copolymers and Blends" by Philippe Demont, et al.).
The principle of the thermal stimulated windowing technique is summarized herewith. The technique has been used a great deal by the scientists of the Laboratory of Physique des Solides in Toulouse, France. These researchers, headed by Professor Lacabanne, concentrated on the application of the thermal windowing method with the aim of isolating one by one the individual relaxations making up a cooperative complex spectrum. The method consists in applying an excitation field (electrical or mechanical) to induce orientation in the material at a selected temperature of excitation T.sub.p. The temperature is subsequently lowered by a few degrees, with the field still applied. At that temperature T.sub.d, the excitation field is then removed and the material is free to return to its state of equilibrium at this temperature T.sub.d. However, it can only do so for a small time t.sub.d and therefore the material cannot relax completely at T.sub.d, and the remaining orientation induced in the material is then frozen in by quenching to a very low temperature T.sub.0. The subsequent reheating at a controlled heating speed, discloses the elementary kinetics of the relaxation mode isolated in the window temperature range (T.sub.p -T.sub.d). The curve obtained during this recovery stage at a constant rate of heating is of a Debye nature, which may be analyzed directly and quantitatively according to the Arrhenius formulation to determine the activation enthalpy and activation entropy parameters for this isolated deconvoluted elementary relaxation. By changing the value of T.sub.p around the global temperature peak observed in either TSC or TSCR, all the relaxation modes co-operating in an interactive manner and contributing to the global response observed without thermal windowing can be isolated one by one. This represents the description of the prior art according to the processes described as thermal stimulated processes.
However, these known methods for analyzing and characterizing materials by the thermal stimulated effect have major negative drawbacks: the method using thermal stimulated current cannot be applied to conductors or semi-conductor materials for which the electrical resistance is smaller than 10.phi. ohms/meter of thickness; the method using thermal stimulated creep is not easy to apply to pasty or liquid materials and does not allow temperatures close to the fusion point of the materials to be reached; and there is no simple correlation between the distribution spectra for the relaxation times obtained by TSC and TSCR analysis. This is a major drawback which casts a doubt on the validity of the results obtained by these techniques. The relationship between the mechanical and electrical spectrum of relaxation appears to be complex. In addition, the thermal stimulated method presented in the prior art appears to disturb the structural state of the sample owing to the very nature of the experiment itself: the TSC or TSCR methods consist in applying an electrical or mechanical field at a temperature T.sub.p in the vicinity of the temperature at which the internal motions occur. The effect of bringing the material to this temperature T.sub.p enables the latter to relax from its internal stresses, if there are any present, or to modify its morphology, if it is capable of crystallizing, or even modifying its degree of curing for curable materials and thermoset resins. It is therefore clear that thermal stimulated processes are restricted to the study of internal motions undisturbed by morphological changes at the analysis temperature T.sub.p.
The main disadvantage of differential microcalorimetry or of differential thermal analysis is that the instrument response is a global response which integrates the co-operative plurality of internal relaxations. A further main disadvantage is the low sensitivity in detecting "secondary" internal movements for which the activation enthalpy is low. Finally, this technique also presents difficulties, especially a lack of sensitivity, in studying certain phenomena such as the orientation of plastic materials or the physical aging phenomena. For instance, it is not rare to observe great variations in the mechanical properties of plastic materials and not to lack such evidence of any difference on the basis of the corresponding traces in DSC analyses. Differential microcalorimetry appears not to be very sensitive to internal stresses relaxed kinetically during physical aging.
Another major disadvantage of the thermal stimulated processes described in the prior art is that the sample must be changed for each temperature T.sub.p when the object of the analysis is to study physical aging or internal stresses. This results in a long and expensive analysis process. In the prior art, a technician using the TSC analysis cell or TSCR analysis cell must prepare a variety of samples and introduce them in succession one after the other. The thermal windowing experiments are then run according to the previous description and a new sample has to be entered into the chamber for each T.sub.p since the sample which has been analyzed has lost its initial condition, which is what is being studied. The above procedure is repeated for each excitation temperature T.sub.p with a new sample until the complete relaxation spectrum is obtained. This method of analysis for isolating simple modes in materials having internal stresses requires a large number of samples and a great deal of labor.
Yet another major disadvantage of the prior art is that the TSC and TSCR cells are different and the two techniques cannot be used simultaneously on the same sample.